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River Dynamics 101 - Fact Sheet
River Management Program
Vermont Agency of Natural Resources
Overview
In the discussion of river, or fluvial systems, and the strategies that may be used in the management of fluvial systems, it is important to have a basic understanding of the fundamental principals of how river systems work. This fact sheet will illustrate how sediment moves in the river, and the general response of the fluvial system when changes are imposed on or occur in the watershed, river channel, and the sediment supply.
The Working River
The complex river network that is an integral component of Vermont’s landscape is created as water flows from higher to lower elevations. There is an inherent supply of potential energy in the river systems created by the change in elevation between the beginning and ending points of the river or within any discrete stream reach. This potential energy is expressed in a variety of ways as the river moves through and shapes the landscape, developing a complex fluvial network, with a variety of channel and valley forms and associated aquatic and riparian habitats. Excess energy is dissipated in many ways: contact with vegetation along the banks, in turbulence at steps and riffles in the river profiles, in erosion at meander bends, in irregularities, or roughness of the channel bed and banks, and in sediment, ice and debris transport (Kondolf, 2002).
Sediment Production, Transport, and Storage in the Working River
Sediment production is influenced by many factors, including soil type, vegetation type and coverage, land use, climate, and weathering/erosion rates. Once the sediment enters the fluvial system it will be transported and/or stored within the system including the flood plains.
The watershed through which a river flows or drains dictates the sediment types and amount, that will be transported and/or stored. Within the watershed there are locations where sediment is produced, transported,or stored. These zones are often referred to as: source (production), transfer (transport), and response (storage or deposition) (Figure1).
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Figure 1 Watershed sediment source, transport and deposition locations. (from the Stream Corridor Restoration Manual, Federal Interagency Stream Restoration Working Group, 1998).
ource Source (^) Transfer Transfer Response Response
- Source streams : Primarily where non- alluvial sediments (colluvial material) enter into the stream system, from landslides and mass wasting failures, and transported with debris during large and infrequent flow events.
- Transfer streams : Geomorphically resilient with high sediment transport capacity. These streams are able to convey limited increases in sediment loads and will change little in response to reduction in sediment supply. Generally, the sediment volume supplied to transport reaches is balanced by the sediment exported from the reach.
- Response streams : Storage reaches in which significant geomorphic adjustment occurs in response to changes in sediment supply. Zones of transition from transport to response or storage reaches are locations where changes in sediment supply may result in both pronounced and persistent channel instability.
After the sediment enters the fluvial system, the movement of sediment is influenced not only by the zone of the watershed the river is in, but also the local conditions of the river. The sediment transport capacity refers to the amount and size of sediment that the river has the ability, or energy to transport. The key components that control the sediment transport capacity, are the velocity and depth of the water moving through the channel. Velocity and depth are controlled by the channel slope and dimensions, discharge (volume of flow), and roughness of the channel. Changes in any of these parameters will result in a change in the sediment transport capacity of the river.
The specific characteristics of the sediment load is another key factor influencing channel form and process. The load is the total amount of sediment being transported. There are 3 types of sediment load in the river: dissolved, suspended, and bed load. The dissolved load is made up of the solutes that are generally derived from chemical weathering of bedrock and soils. Fine sands, clay, and silt are typically transported as suspended load. The suspended load is held aloft in the water column by turbulence. The bed load is made up of sands, gravel, cobbles, and boulders. Bed load is transported by rolling, sliding, and bouncing along the bed of the channel (Allan, 1995). While dissolved and suspended load are important components of the total sediment load; in most river systems, the bed load is what influences the channel morphology and stability (Kondolf, 2002).
By comparing the sediment transport capacity with the sediment load, some general assumptions can be made as to whether a river will erode more sediment, deposit extra sediment, or be in balance with the amount of erosion and deposition happening. For example:
- If the capacity is greater than the load, erosion would be expected. {capacity > load = erosion}. This is due to the river having the excess energy needed to transport more sediment than is currently being transported.
- If the capacity is less than the load, deposition would be expected {capacity < load = deposition}. The amount of excess energy needed to move the extra sediment is not available in the system, so the sediment is deposited in the channel.
- If the capacity equals the load, no net change in erosion and deposition would be expected. {capacity = load = no net erosion/deposition}. River systems, or reaches ,are considered in equilibrium when there is a balance between the amount of sediment load being supplied to the system and the capacity of the system to carry that sediment load (Field, 2002).
Another way to view this concept is to use Lane’s Diagram (Figure 2). Lane’s balance diagram demonstrates how the channel may respond to a change in various parameters, such as sediment load, channel geometry, channel slope, erosion resistance, and discharges (hydrologic load). For example, by increasing the amount of sediment load the scale will tip toward aggradation (sediment deposition); to bring the scale back in balance a change in either the channel geometry, slope and/or hydrologic load would be needed. There are natural fluctuations in the balance of any of these inputs; such as flood events, valley wall slope failures increasing sediment loads, beaver dams or debris jams causing changes in channel geometry, etc. Human caused changes in this balance are also occurring in the watershed, along the floodplains, and in the channel. The degree and type of adjustment will depend on whether it is a source, transfer or response reach, the sediment transport capacity, and the type and magnitude of change that was introduced to the system.
(Energy / Power) Channel Geometry
(Boundary Condition) Sediment Size
Watershed
Input:
Hydrologic
Load
Watershed
Input:
Sediment
Load
Figure 2 : Lane’s Diagram (1955) from Rosgen 1996
Types of Channel Adjustments in the Working River
When the balance of sediment load, hydrologic load, and/or channel geometry and slope is changed there is often a response, or adjustment of the fluvial system as it attempts to re-establish the equilibrium condition. Of the types of rivers typically seen in Vermont, the process of adjustment is predictable. Schumm’s Channel Evolution Model (1984) in Figure 3 illustrates the most commonly encountered channel adjustment or response sequence in Vermont. The channel evolution model is helpful in demonstrating the response of the river to climate driven or human imposed changes within its watershed, floodplain, and within the channel itself. It is also valuable in demonstrating that rivers are not static systems; rivers will respond and adjust as the input variables to the system are changed.
Rivers are a metaphor for “change.” Every fluvial system changes in time. Sensitivity refers to the likelihood that a stream reach will respond to a watershed or local disturbance. The exercise of assigning a sensitivity rating to a stream is done in the context that some streams, due to their setting and location within the watershed, are more likely to be in an episodic, rapid, and/or measurable state of change or adjustment. A stream’s inherent sensitivity may be heightened when human activities alter the setting characteristics that influence a stream’s natural adjustment rate including: boundary conditions; sediment and flow regimes; and the degree of confinement within the valley. Streams that are currently in adjustment, especially degradation or aggradation, may be acutely sensitive.
The Dynamic River
Knowing that rivers are a dynamic system and are sensitive to change (Figure 4), the types of management strategies considered for the watershed and the river must evaluate the short and long term effect(s) of that management strategy on the fluvial system. The geomorphic condition of the stream can be used as both an indicator and predictor of watershed function and health. Applying the science provides tools to manage fluvial conflicts between river processes and human investments by recognizing streams as a continuous system, rather than a collection of unrelated problem sites.
DEPTH, SLOPE & FLOODPLAIN HOW STREAMS WORK ARE CRITICAL FACTORS IN: È Dynamic equilibrium is achieved through sediment continuity (Sediment In = Sediment Out)
È Sediment continuity is achieved when the stream has the power to move the size and quantity of DIMENSION sediment delivered to the stream reach
È Stream power is a function channel slope and discharge
È Channel slope is a function of floodplain width and meander pattern
È Flow velocity and channel area are functions of PATTERN & PROFILE discharge and are determined by channel and floodplain dimensions (width and depth)
Figure 4. “How Streams Work” to achieve sediment continuity and dynamic equilibrium.
References :
1. Allan, J.D. 1995. Channels and flow, the transport of materials, in Stream ecology, structure and function of running waters. Chapman & Hall Publishing, London. p.8-
2. Center for Watershed Protection et.al. 1999b. Watershed Hydrology Protection and Flood Mitigation Project Phase II-Technical Analysis. Prepared for Vermont Geological Survey. Vermont ANR. Waterbury, VT.
3. Field, J. 2002. Fluvial geomorphology short courses. Field Geology Services Course 1. 4. Kondolf, G.M., M. Smeltzer, and L. Kimball. 2002. Freshwater Gravel Mining and Dredging Issues. Prepared for Washington Department of Fish & Wildlife, Department of Ecology, and Department of Transportation. p.3-4.
5. Lane, E.W. 1955. The Importance of Fluvial Morphology in Hydraulic Engineering, American Society of Civil Engineering, Proceedings, 81, paper 745: 1-17.
6. Rosgen D. 1996. Applied Fluvial Morphology. Wildland Hydrology Books, Pagosa Springs, Co
7. Schumm, S. 1984. The Fluvial System, John Wiley and Sons, New York.
8. Vermont Agency of Natural Resources. “Vermont Stream Geomorphic Assessment Phase 3 Handbook”, prepared by the DEC River Management Program , April 2004. Protocol handbook can be obtained at http://www.watershedmanagement.vt.gov/rivers/htm/rv_geoassess.htm
